1. Field of the Invention
The present invention relates in general to a lubricating device for a power transmission system of a motor vehicle wherein all rotating members are at rest upon stopping of the vehicle, and more particularly to such a lubricating device utilizing a mechanical pump which is driven by the power transmission system.
2. Discussion of the Prior Art
A power transmission system such as a transmission, a speed reducing device or a differential gear device used in a motor vehicle is equipped with a lubricating device for lubricating various rotating members of the power transmission system so as to protect bearing portions and mutually meshing portions of the rotating members from seizure and early wearing. There are proposed various types of such a lubricating device for the vehicle power transmission system. As one example of the lubricating device, a so-called "oil bath lubrication" is known, wherein portions of the power transmission system are immersed in a lubricating oil stored in a suitable oil pan, and the lubricating oil is scattered by the rotating members within the power transmission system during operation. Another example of the lubricating device is known as a so-called "forced-feed lubrication", wherein the lubricating oil stored in the oil pan is sucked and pressurized by a suitable oil pump, and the pressurized oil is delivered to the lubricating points such as the bearing and meshing portions of the rotating members. In a lubricating device as disclosed in laid-open Publication No. 5-89061 of unexamined Japanese Utility Model Application, the power transmission system is subject to the oil bath lubrication when the running speed of the vehicle is relatively low, while on the other hand, the power transmission system is subject to the forced-feed lubrication as well as the oil bath lubrication when the running speed of the vehicle is relatively high. Further, two types of the forced-feed lubrication are known, i.e., mechanical forced-feed lubrication in which the pump for delivering the lubricating oil is driven by a member of the power transmission system, and an electrically- operated forced-feed lubrication in which an electrically operated pump is employed to deliver the lubricating oil.
In the oil-bath lubrication as described above, the stored lubricating oil is agitated and scattered by the rotating members of the power transmission system when these members are rotated. In this case, the vehicle undesirably suffers from an energy loss especially when the vehicle is running at a relatively high speed, that is, when the rotating members of the power transmission system are rotated at a relatively high speed. When the oil bath lubrication is used in an electric vehicle, for example, the travel distance of the vehicle per one battery charging is undesirably reduced. Further, since the lubricating oil is agitated intensively by the rotating members, the oil is apt to be overheated, leading to early deterioration of the oil. Moreover, as a result of the agitation of the oil by the rotating members, bubbles likely to be formed in the lubricating oil and the transmission system tends to suffer from an oil leakage.
In some electric vehicle, the vehicle speed is controlled exclusively by the rotating speed of a drive motor of the vehicle. In this case, however, the rotating speed of the input shaft of the power transmission system is extremely high during running of the vehicle at a high speed, and the power transmission system cannot necessarily be lubricated to a satisfactory extent by the oil bath lubrication.
In the lubricating device as disclosed in the above publication, the power transmission system is subject to the forced-feed lubrication in addition to the oil bath lubrication while the vehicle is running at a relatively high speed. This arrangement is capable of assuring a sufficient lubricating effect during running of the vehicle at a relatively high speed. Further, the amount of the lubricating oil used for the oil bath lubrication can be decreased, and the energy loss due to the agitation of the oil by the rotating members is accordingly reduced. However, as long as the power transmission system is subject to the oil bath lubrication while the vehicle is running at a high speed, the vehicle suffers from an energy loss due to the agitation of the oil by the rotating members of the power transmission system. Thus, the disclosed lubricating device is not necessarily satisfactory when used in the electric vehicle in which the rotating speed of the power transmission system is extremely high when the vehicle running speed is relatively high and in which an improvement of the travel distance per one battery charging is required.
Unlike the oil bath lubrication, the above-described mechanical forced-feed lubrication is not likely to cause the energy loss. However, in the electric vehicle, all of the rotating members of the power transmission system are turned off when the vehicle is stopped. Accordingly, a mechanical oil pump which is adapted to be driven by the rotation of the power transmission system is also turned off, and therefore, the lubricating oil is no more supplied to the power transmission system. This means that the power transmission system cannot be adequately lubricated upon restarting of the vehicle after a stop. Described in detail, upon starting of the vehicle, the power transmission system starts to be rotated and accordingly, the lubricating oil is delivered by the mechanical pump and gradually supplied to the power transmission system. In the actual operating state of the vehicle, however, it takes some time before the delivered oil reaches the power transmission system. Furthermore, the rotating speed of the power transmission system is usually relatively low upon and immediately after starting of the vehicle, and accordingly, the amount of the lubricating oil delivered from the pump is small. However, a large amount of the lubricating oil is required to sufficiently lubricate the power transmission system upon and immediately after starting of the vehicle, since a comparatively large torque is transmitted to the power transmission system via the input shaft thereof and a relatively large amount of load acts on the bearing and meshing portions of the power transmission system. In case that the vehicle wheels slip on the road surface upon starting of the vehicle, the gears of the differential gear device are rotated at a high speed with a large amount of load applied thereto, and therefore a large amount of the lubricating oil is required to lubricate the gears. If the gears are not adequately lubricated, they would be damaged due to shortage of the lubricating oil.
In the electrically-operated forced-feed lubrication, the electric pump is capable of delivering the lubricating oil to the power transmission system irrespective of the rotating speed of the power transmission system. Thus, the electrically-operated forced-feed lubrication is preferably employed in the electric vehicle wherein all of the rotating members of the power transmission system are turned off upon stopping of the vehicle. However, the electrically-operated forced-feed lubrication suffers from various problems as described below. For example, the electrically-operated forced-feed lubrication needs an electric motor for driving the electric pump, pushing up the cost of manufacture of the lubricating device as compared with the above-described mechanical forced feed-lubrication. Further, components of the electric motor such as a brush have a comparatively shorter service life than the vehicle itself, and they should be replaced or checked regularly. This regular replacement and checking make the vehicle maintenance cumbersome. To obtain a sufficient lubricating effect upon starting of the vehicle in the lubricating device employing the electric pump, the electric pump motor must be kept in the operating state for driving the electric pump so as to enable the lubricating oil to be continuously delivered to the power transmission system even while the vehicle is at a stop, since it is not possible to anticipate when the accelerator pedal is depressed by the vehicle driver for restarting the vehicle. In this case, the electric power consumed by the electric motor for continuously driving the electric pump is generally larger than the energy loss experienced in the mechanical forced-feed lubrication wherein the mechanical pump is driven by the rotation of the power transmission system.